1. Field of the Invention
This invention relates to a method of controlling a compression ignition internal combustion engine in which a mixture of air, and fuel, such as gasoline or gas oil, is burned by compression ignition.
2. Description of the Prior Art
In general, in a compression ignition internal combustion engine, fuel is burned at a higher compression ratio and at a larger or leaner air-fuel ratio (subjected to lean burn) than in a spark ignition internal combustion engine, whereby excellent fuel economy and heat efficiency are attained. Conventionally, a method of controlling this kind of compression ignition internal combustion engine was proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 9-287528. According to this control method, when the engine is in a low-load operating condition, intake air is heated by an electric heater arranged in an intake pipe, whereby the temperature of gases or air taken in is increased for quicker ignition of an air-fuel mixture supplied to the engine.
The heating of intake air is carried out for the following reason: In the compression ignition internal combustion engine, during low-load operation of the engine, such as idling, the temperature of the compression chamber is decreased and hence the timing of ignition is largely retarded, resulting in an unstable combustion of the engine. Particularly when a gasoline having a high octane value is used as fuel, a misfire can occur eventually. Therefore, according to this control method, when the engine is in a low-load operating condition, to ensure stability of combustion of the engine, intake air is heated e.g. by the above-mentioned electric heater to advance the timing of ignition of the mixture.
According to the above conventional control method, it is required to operate the electric heater to maintain stability of combustion during the low-load operation of the engine, and hence consume an increased amount of power, which results in an increase in running costs. At the same time, manufacturing costs of the internal combustion engine are also increased by the cost of provision of the electric heater. Further, in the compression ignition internal combustion engine, it is difficult to ignite the mixture by compression ignition when the engine is started at a low temperature, so that it is required to operate the electric heater for a long time period and set the compression ratio to a considerably high value so as to ensure excellent startability. However, if the electric heater is operated over such a long time period, power consumption is still more increased. Further, to set the compression ratio to the considerably high value, it is required to increase thickness of operating components such that strength thereof is ensured, and enhance sealing properties between pistons and cylinders. The resulting increase in inertial mass of the operating components increases inertial resistance, and the enhanced sealing properties of the associated components increase friction therebetween, which results in lower fuel economy.